This invention relates to distributed receiver systems associated with antenna arrays and especially to the calibration of such receiver systems.
Arrays of antennas are used when it is desired to detect small signal strength, for example, in the case of a high frequency (approximately between 3 MHz and 30 MHz) radar installation. Receiving antenna arrays which could be suitable for detecting surface or sky wave might have many antenna elements spaced apart to form a long antenna aperture (typically between tens of meters to several thousand meters).
From signals appearing at the antenna terminals narrow receiving beams are formed, usually by means of digital computation, after the weak antenna signals are amplified by frequency selective receiving equipment then sampled and converted into digital signals. The advantages of digital beamforming are maximised when one receiving antenna element is feeding one and only one receiver, i.e., each receiver is dedicated to a specific antenna element.
Cables connecting the antenna elements to the receivers (or to pre-amplifiers if they are physically separated from the receivers) are usually made physically short in order to minimise signal loss due to cable attenuation. Therefore, the installed receiving system (that is the collection of receiving apparatus and associated supporting peripherals such as local oscillators, timing units, frequency and timing distributors, pre-amplifiers, signal pre-processors, interfaces etc.) will become distributed along the physical aperture of the antenna array. The receiving equipment on the receiving site might be evenly distributed or clustered in more than one shelter.
Beamforming techniques by digital computation are well known from the technical literature. Most beamforming computation in essence involves the multiplication of the digitised signal samples from each of the receiver outputs with the beam coefficients followed by summing these products for corresponding signal samples. One set of beam coefficients is specific to a given beam pointing direction and as many sets are required as number of beams to be formed.
The theoretical values of beam coefficients assume equal signal transfer between antenna terminals and associated receiver outputs for all the elements in the receiving array. Should the actual receivers differ in their transfer functions then the beam coefficients must be corrected by calibration factors, so that the resultant beam(s) will satisfy the beamwidth and sidelobe level requirements.
Practical receivers made to some manufacturing tolerances may differ in their initial electrical characteristics and are subject to further variation in use. When integrated into a system, changes can take place in the receiver itself and/or in the auxiliary input signals to the receiver. For example, fixed and variable local oscillator frequencies (generated centrally in the system and distributed to the receiver mixers) might change in amplitude and phase and cause a corresponding effect in the received signal. Furthermore, changes in the power supply and in the ambient temperature will have indirect effects on the signal.
The time dependent changes in a receiver's transfer characteristic is observable in a slow random variation in amplitude, phase and group delay of the output signal. For example, if the same signal was applied to the inputs of all receivers in a distributed system then, at a given time, the output signal's amplitude and phase would be unlikely to remain identical but, instead, be distributed randomly between the receivers with a finite variation. The apparent random distribution can be expected to change with time to other random distributions.
The objective of a calibration procedure is to determine the receiver's transfer characteristics for the signal components of the used waveform. Waveforms, in general, can be viewed as being composed from a collection of sinusoidal waves each of which is described by a complex number with parameters of amplitude and phase at a given frequency.
Calibration should be carried out for less than or equal to that time interval which corresponds to just tolerable errors in the formed beams resulting from waveform component variations in the receiver system over that interval. In order to maximise operation time, the calibration procedure must be rapid and efficient.
For example, one possible calibration procedure for the receiving system would involve the disconnection of the receiver cables from the antenna elements and feeding test signals into the receiver inputs. Measurements of the output signal could be carried out one by one for each receiver in order to obtain a set of calibration data. While such a consecutive method would be adequate for installations with small number of receivers, for a large aperture distributed system the calibration time requirement would reduce prohibitivety the system availability for operation.
Concurrent measurements would require a distribution network for delivering the test signal to receivers which might be spaced out over several thousand meters. Such a network must ensure that the test signals at all receiver input terminal are identical in both amplitude and phase at any frequency. Clearly the test signal distribution network (purely passive or possibly containing active components) will require initial setting up and periodic calibration, as its components, similarly to the main receiver system, are subject to time dependent variation. Calibration of such large scale distribution network would create problems that are comparable with the receiving system calibration.